X-rays radiation has wide applications in industry nondestructive testing, security inspection and other domains. For large objects to be tested, e.g., boilers, airspace engines, mass cargoes at airports/railway stations/customs, high energy X-rays are required for their fluoroscopy examinations, generally produced by electron accelerators at energies above 2 MeV. The basic way of producing X-rays by an electron accelerator is as follows: producing an electron beam with an electron gun, accelerating the electron beam by an electric field to obtain high energy, shooting a target by the high energy electron beam to produce X-rays. X-rays produced by the electron beam shooting on the target generally distribute in various direction at a 4π solid angle. X-rays produced by electron beams with different energies shooting on a target may have different intensity distributions in various radiation directions. In general, the higher energy an electron beam has, the higher intensity the forward X-rays have, and the lower intensity the X-rays in the other directions have. With a target point as the center and X-ray intensity in various directions as amplitude, the X-ray intensity has a “pine core shaped” distribution over solid angles as shown in FIG. 3. In various X-ray fluoroscopy imaging systems, instead of adopting all X-rays over the 4π solid angle, only a small portion is adopted. In many situations, X-rays in a planar fan-shaped area are used, such as a sliced “planar fan-shaped beam”. As shown in FIG. 2, such a system is composed by a high energy electron accelerator, a shielding and collimating device, a detector and a signal analysis and image processing system, with dotted arrows schematically showing a “planar fan-shaped beam”. “Planar fan-shaped beams” have wide applications in high energy industry CT, container inspection systems, vehicle inspection systems, train fluoroscopy examination systems, air cargo inspection systems, luggage and parcel inspection systems and the like.
In existing methods for obtaining a “planar fan-shaped beam” in the prior art, a “thin gap” shielding collimator is arranged in front of the target of the high energy electron accelerator to shield X-rays in most of directions while only passing X-rays through the “thin gap” to form a “planar fan-shaped beam”. In general, the “thin gap” is arranged directly in front of the target, and on the advancing direction of the shooting electron beam, which is referred to as “zero degree” direction herein. The highest intensity of X-rays produced by electron accelerators is used in the prior art. However, there are large differences between X-ray intensities in different directions in the fan area. Taking a 9 MeV accelerator as an example, the X-ray intensity in the zero degree direction of the fan area is 10 times of that in the 45-degree direction at the edge. After many years of development of the X-ray fluoroscopy imaging system shown in FIG. 2, there is a great improvement on the performance of the detector, and the demand for the intensity of X-rays has been significantly reduced. However, in order to improve image quality, there is an increasing requirement for the quality of X-rays. For example, a ratio of intensity differences of X-rays in various directions in the fan area, of less than 2, i.e., a “planar fan-shaped beam having uniform intensity”, is desired.